<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <form>
        <input type="text" autofocus>
        <input type="submit" value="提交">
    </form>

    <form>
        <input type="text" placeholder="请输入用户名:">
        <input type="submit" value="提交">
    </form>

    <form>
        <input type="text" required="请填写此字段">
        <input type="submit">
    </form>
</body>


</html>